1. Field of the Invention
The present invention relates to a hydrogen storage alloy electrode which can be used as a negative electrode for alkaline storage batteries and also to a method for manufacture of the hydrogen storage alloy electrode.
2. Description of Related Art
With the recent rapid expansion of the nickel-hydrogen storage battery market, there is an increasing demand to extend the lives of nickel-hydrogen storage batteries.
One of the factors that influence the lives of nickel-hydrogen storage batteries is the corrosion of hydrogen storage alloy particles for use as negative active material. Therefore, the improvement in corrosion resistance of hydrogen storage alloy electrodes appears to be a key solution to extending the lives of nickel-hydrogen storage batteries.
Japanese Patent Laying-Open No. Hei 10-106550 (1998) proposes a method of improving corrosion resistance of a hydrogen storage electrode by dispersing yttrium oxide or hydroxide therein to thereby suppress corrosion of hydrogen storage alloy particles in alkaline electrolytes.
However, the inventors of the present application have found from their investigations that the method described in the above-identified reference results not only in failure to provide an appreciable improving effect, but also in the reduction in hydrogen storing capacity of a hydrogen storage alloy electrode leading to build-up of a battery""s internal pressure during charge.
It is an object of this invention to provide a hydrogen storage alloy electrode having improved corrosion resistance which, when incorporated in a storage battery, can improve its charge-discharge cycle performance and suppress build-up of its internal pressure during charge. It is a further object of this invention to provide a method for manufacture of such a hydrogen storage alloy electrode.
A hydrogen storage alloy electrode of this invention contains, as its principal active material, a powder of hydrogen storage alloy having a CaCu5 crystal structure and represented by the formula MmNixCoyMnzMw, where M is at least one element selected from aluminum (Al) and copper (Cu), x is between 3.0 and 5.2 (3.0xe2x89xa6xc3x97xe2x89xa65.2), y is between 0 and 1.2 (0xe2x89xa6yxe2x89xa61.2), z is between 0.1 and 0.9 (0.1xe2x89xa6zxe2x89xa60.9), w is between 0.1 and 0.9 (0.1xe2x89xa6wxe2x89xa60.9), and the sum of x, y, z and w is between 4.4 and 5.4 (4.4xe2x89xa6x+y+z+wxe2x89xa65.4). The hydrogen storage alloy powder consists of particles each having a surface region and a bulk region enclosed within the surface region. The powder particle has a higher nickel content in the surface region than in the bulk region. The electrode further contains an oxide and/or hydroxide of at least one rare-earth element selected from ytterbium (Yb), samarium (Sm), erbium (Er) and gadolinium (Gd).
As stated above, the hydrogen storage alloy powder particles have a surface region and a bulk region enclosed within the surface region. The surface and bulk regions of the hydrogen storage alloy powder particle can be defined in terms of nickel contents. That is, when a nickel content of the powder particle is measured along a line extending from a surface toward a center, its values become substantially constant at any location of measurement beyond a certain point. A series of such points makes a boundary. A region that extends from the boundary toward the surface can be defined as the surface region. A region inside the boundary can be defined as the bulk region where the powder particle has a substantially constant nickel content.
Specifically, the numbers of all constituent element atoms and oxygen atoms in the powder particle are measured at locations spaced at intervals of several nanometers from its surface toward its center by utilizing a scanning transmission electron microscope and an energy dispersive X-ray spectroscope. Then, a nickel content (atomic percent) can be calculated from a proportion in number of nickel atoms to all constituent element atoms and oxygen atoms.
The boundary at which a nickel content converges to a constant value can be determined from results obtained via the above-described procedure. Once it is determined, the surface region and the bulk region can be defined in the manner as described above.
In the present invention, the powder particle has a higher nickel content in the surface region than in the bulk region. As stated above, the nickel content of the powder particle is substantially constant in the bulk region and is varied in the surface region. In this specification, the xe2x80x9cnickel content in the surface regionxe2x80x9d means the nickel content at an intermediate depth of the surface region, i.e., the nickel content measured at a location intermediate between the outermost surface and the boundary. Preferably, the nickel content is at least 1.1 times as high in the surface region as in the bulk region.
Characteristically, a hydrogen storage alloy electrode of this invention contains the aforementioned hydrogen storage alloy powder, as its primary active material, and an oxide and/or hydroxide of at least one of the above-listed rare-earth elements. Inclusion of such an oxide and/or hydroxide of rare-earth element increases the corrosion resistance of the hydrogen storage alloy electrode to alkaline electrolytes, resulting in the improved charge-discharge cycle performance. Since the increased corrosion resistance makes it difficult for the hydrogen storage alloy electrode to undergo electro-chemical property reduction in the alkaline electrolyte, the build-up of a battery""s internal pressure during charge can be suppressed. As used herein, xe2x80x9coxide and hydroxide of a rare-earth elementxe2x80x9d refer to those containing the rare-earth element in the cation form.
Preferably, the aforementioned oxide and/or hydroxide of a rare-earth element are contained in the electrode in the amount of 0.1-5.0 parts by weight, based on 100 parts by weight of the hydrogen storage alloy powder. Inclusion thereof within the above-specified range further improves a battery""s charge-discharge cycle performance and suppresses the build-up of a battery""s internal pressure during charge more effectively.
Details are not clear as to how the hydrogen storage alloy electrode of this invention exhibits the increased corrosion resistance to alkaline electrolytes. It is believed, however, that the intrinsic alkaline electrolyte corrosion resistance of the hydrogen storage alloy powder particle in its surface region is markedly enhanced by deposition thereon of the oxide and/or hydroxide of a rare-earth element.